The Eastern Marsh Harrier (Circus spilonotus) is a raptor of the family Accipitridae that breeds in northeastern Eurasia and migrates southward for the winter. In Japan, it appears mainly as a winter visitor. The species measures approximately 48–58 cm in length. Males have more pale areas, while females are predominantly brown; both sexes show considerable individual variation in overall appearance. The wings are long and broad, forming a gentle, characteristic V-shape in flight.
A defining feature of the species is its low, unhurried glide over reed beds and wetlands, where it searches for prey. It often patrols close to the ground at slow speed, taking small mammals, birds, amphibians, and reptiles. During the breeding season, the species is known to perform aerial courtship displays in midair.
Extensive reed beds and wetland habitats are essential for sustaining this species, yet habitat loss due to development has become a growing concern. The presence of the Eastern Marsh Harrier is regarded as an indicator of a healthy natural environment, and ongoing habitat conservation efforts remain vital for its future.